Near-Earth supernova

A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth, less than roughly 10 to 300 parsecs [33 to 978 light-years] away, to have noticeable effects on its biosphere.

An estimated 20 supernova explosions have happened within 300 pc of the Earth over the last 11 million years. Type II supernova explosions are expected to occur in active star-forming regions, with 12 such OB associations being located within 650 pc of the Earth. At present, there are 12 near-Earth supernova candidates within 300 pc.

On average, a supernova explosion occurs within 10 parsecs (33 light-years) of the Earth every 240 million years.^{[citation needed]} Gamma rays are responsible for most of the adverse effects that a supernova may have on a life-harboring terrestrial planet. In Earth's case, gamma rays induce radiolysis of diatomic N₂ and O₂ in the upper atmosphere, converting molecular nitrogen and oxygen into nitrogen oxides, and thereby notably depleting the ozone layer, enough to expose the surface to bio-harmful (mainly ultra-violet) solar and cosmic radiation. Phytoplankton and reef communities would be particularly affected, which could severely deplete the base of the marine food chain.

Historically, nearby supernovae may have influenced the biodiversity of life on the planet. Geological records suggest that nearby supernova events have led to an increase in cosmic rays, which in turn produced a cooler climate. A greater temperature difference between the poles and the equator created stronger winds, increased ocean mixing, and resulted in the transport of nutrients to shallow waters along the continental shelves. This led to greater biodiversity.

Odenwald discusses the possible effects of a Betelgeuse supernova on the Earth and on human space travel, especially the effects of the stream of charged particles that would reach the Earth about 100,000 years later than the initial light and other electromagnetic radiation produced by the explosion. However, it is estimated that it may take up to 1.5 million years for Betelgeuse to undergo a supernova.

Speculation as to the effects of a nearby supernova on Earth often focuses on large stars as Type II supernova candidates. Several prominent stars within a few hundred light years of the Sun are candidates for becoming supernovae in as little as 1,000 years. Although they would be extremely visible, if these "predictable" supernovae were to occur, they are thought to pose little threat to Earth.^{[citation needed]}

It is estimated that a Type II supernova closer than eight parsecs (26 light-years) would destroy more than half of the Earth's ozone layer. Such estimates are based on atmospheric modeling and the measured radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from 0.05–0.5 per billion years to 10 per billion years. Several studies assume that supernovae are concentrated in the spiral arms of the galaxy, and that supernova explosions near the Sun usually occur during the approximately 10 million years that the Sun takes to pass through one of these regions. Examples of relatively near supernovae are the Vela Supernova Remnant (c. 800 ly, c. 12,000 years ago) and Geminga (c. 550 ly, c. 300,000 years ago).

Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place in a star system that is not well studied. The closest known candidate is IK Pegasi. It is currently estimated, however, that by the time it could become a threat, its velocity in relation to the Solar System would have carried IK Pegasi to a safe distance.